The present invention relates to an engine exhaust purification device provided with a catalyst
JP-A-H9-228873 published by the Japanese Patent Office in 1997 discloses a technique wherein an oxygen amount stored in a three-way catalyst (hereafter, xe2x80x9coxygen storage amountxe2x80x9d) is computed based on an engine intake air amount and an air-fuel ratio of an exhaust flowing into the catalyst, and engine air-fuel ratio control is performed so that the oxygen storage amount of the catalyst is constant.
To maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon) conversion efficiency of the three-way catalyst at a maximum, the catalyst atmosphere must be maintained at the stoichiometric air-fuel ratio. By maintaining the oxygen storage amount of the catalyst constant, oxygen in the exhaust is stored in the catalyst when the air-fuel ratio of the exhaust flowing into the catalyst shifts to lean, and oxygen stored in the catalyst is released when the air-fuel ratio of the exhaust flowing into the catalyst shifts to rich, so the catalyst atmosphere can be maintained at the stoichiometric air-fuel ratio.
In an exhaust purification device which performs this control, the conversion efficiency of the catalyst depends on the oxygen storage amount of the catalyst. Therefore, to control the oxygen storage amount to be constant and maintain the conversion efficiency of the catalyst at a high level, the oxygen storage amount must be precisely computed.
However, in the prior art computational method, the oxygen amount already stored on engine startup was not considered, and it was difficult to precisely compute the oxygen storage amount of the catalyst.
It is therefore an object of this invention to further increase the precision of computing the oxygen storage amount in the aforesaid purification device technique. In particular, it is an object of the invention to increase computational precision immediately after engine startup, and to maintain the conversion efficiency of the catalyst at a high level even immediately after engine startup.
In order to achieve above object, this invention provides an exhaust purification device for an engine, comprising a catalyst provided in an exhaust passage of the engine, a first sensor which detects the characteristics of the exhaust flowing into the catalyst, and a microprocessor. The microprocessor is programmed to estimate an oxygen storage amount of the catalyst on engine startup based on the temperature of the catalyst on engine startup, compute the oxygen storage amount of the catalyst based on the detected exhaust characteristics, using the oxygen storage amount on engine startup as an initial value, and control the air fuel ratio of the engine based on the computed oxygen storage amount so that the oxygen storage amount of the catalyst is a target value.
According to an aspect of this invention, this invention provides a method of estimating an oxygen storage amount of a catalyst provided in an exhaust passage of an engine. The method comprises estimating the oxygen storage amount of the catalyst on engine startup based on the temperature of the catalyst on engine startup, and computing the oxygen storage amount of the catalyst based on the characteristics of the exhaust flowing into the catalyst, using the oxygen storage amount on engine startup as an initial value.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.
Strictly speaking, noble metals adsorb oxygen in the molecular state, and oxygen storage materials absorb oxygen as compounds, but in the following description, adsorption and absorption will be collectively referred to as storage.
Further, the expression xe2x80x9cthe exhaust air-fuel ratio is richxe2x80x9d means that the oxygen concentration in the exhaust is lower than the oxygen concentration in the exhaust when the engine is running at the stoichiometric air-fuel ratio, and the expression xe2x80x9cthe exhaust air-fuel ratio is leanxe2x80x9d means that the oxygen concentration in the exhaust is higher than the oxygen concentration when the engine is running at the stoichiometric air-fuel ratio. The expression xe2x80x9cthe exhaust air-fuel ratio is stoichiometricxe2x80x9d means that the oxygen concentration in the exhaust is equal to the oxygen concentration of the exhaust when the engine is running at the stoichiometric air-fuel ratio.